1. Field of the Invention
This invention relates to apparatus for grinding twist drills each having clearance surfaces and a helical flute associated with each clearance surface, which apparatus comprises a rotatable drive shaft, a grinding wheel having a grinding surface and rigidly connected to said drive shaft, and a drill-guiding body, which faces said grinding surface and is axially fixed relative thereto and has guiding passages differing in diameter and adapted to guide clearance surfaces of respective ones of said twist drills differing in diameter toward said grinding surface, also comprising in alignment with each of said guiding passages at least one guide projection for extending into a flute of a twist drill extending through said guiding passage, wherein all guide projections are carried by a guide projection carrier which is disposed between the drill-guiding body and the grinding surface.
2. Description of the Prior Art
Such apparatus for grinding twist drills which need not be chucked is known from U.S. Pat. No. 3,742,652. In that known apparatus the guide projection carrier is axially fixed and is pivotally movable about the axis of the drive shaft for adjusting the angular position of the guide projections relative to the guiding passages.
In a similar grinding apparatus shown in FIG. 1b of U.S. Pat. No. 3,753,320 the guide projections are rigid with a drill-guiding body, which is axially adjustable relative to the grinding surface so that the distance from the guide projections to the grinding surface can be adjusted.
In such apparatus the result of the grinding operation will decisively depend on the proper position of the clearance surface being ground relative to the grinding surface. The position of the clearance surface will depend on the angular position of the guide projection relative to the guiding passage and, owing to the helical flutes of the twist drill, also on the axial distance from the guide projection to the grinding surface.
One disadvantage of said known apparatus resides in that the axial distance from the grinding surface to the guide projections increases with the wear of said grinding surface so that the correct axial distance between the guide projections and the grinding surface must be readjusted (U.S. Pat. No. 3,753,320) or the radial position of the guide projections relative to the guide passages must be changed (U.S. Pat. No. 3,742,652).
Another disadvantage will arise during a prolonged grinding of twist drills which are so small in diameter that they will not contact the entire grinding surface during each revolution of the grinding wheel and resides in that a groove will then be formed in the grinding surface. When a twist drill larger in diameter is subsequently ground, that groove will be reflected by the clearance surface that has been ground, i.e. by the cutting edge of such larger drill, and the grinding wheel cannot be used any longer when such groove has reached a certain size.